Characterization the effects of structure and energetics of intermolecular interactions on the oligomerization of peptides in aqueous 2, 2, 2-trifluoroethanol via circular dichroism and nuclear magnetic resonance spectroscopy.

Intermolecular interactions are of fundamental importance to fully comprehend a wide range of protein behaviors such as oligomerization, folding and recognition. Two peptides, NPY([18-36]) and NPY([15-29]), segmented from human neuropeptide Y (hNPY), were synthesized in this work to study the interaction between species. Information about intermolecular interactions was extracted from their oligomerizing behaviors. The results from CD and NMR showed that the addition of 2, 2, 2-trifluoroethanol (TFE) induces a stable helix in each peptides and an extended helix in NPY([18-36]), formed between residues 30-36. Pulsed field gradient NMR data revealed that NPY([15-29]) forms a larger oligomer at lower temperatures and continuously dissociates into the monomeric form with increasing temperature. NPY([18-36]) was also found to undergo an enhanced interaction with TFE and a more favorable self-association at higher temperatures. We characterized the changes of oligomerized states with respect to temperature to infer the effects of entropy and interaction energy on the association-dissociation equilibrium. As shown by NPY([15-29]), deletion of helical secondary structure or residues from the C-terminal segment may disrupt the solvation by TFE and results in entropy increase as the oligomer dissociates. Unlike that in NPY([15-29]), the extended helix in NPY([18-36]) improves the binding of TFE, and as a result, entropy is gained via the transfer of the TFE cluster from the interface between monomeric peptides into the bulk solvent. This observation suggests that the oligomerized state may be modulated by the entropy and energetics contributed by helical segments in the oligomerization process.